In order to be able to stably supply city water compliant with water quality standards, a water purification technique resulting from selecting and combining various methods is applied in water supply facilities, where the various methods are selected and combined by giving consideration to raw water quality, management goals for purified water quality, the scale of water purification facilities, operation control, the control level of maintenance and management techniques, and the like. For example, the selection is made from among a sterilization-only method, a slow filtration method, a rapid filtration method, and a membrane filtration method, and advanced water purification or the like is combined therewith (Non Patent Literature 1) as required.
Today, the rapid filtration method is adopted by approximately 75% (water volume ratio) of water services because of tolerance to high turbidity and more than a certain level of raw water contamination, because of no need for a vast area of land, because of effectiveness, and so forth.
A water purification plant which adopts the rapid filtration method includes a mixing basin generally injected with a flocculant and used to perform rapid stirring, a flocculation basin used to grow aggregates (flocs), a sedimentation basin used to sediment and remove the grown flocs, and a filter basin used to remove non-settled particles and flocs.
In water purification, chemicals such as powdered activated carbon and disinfectant (liquid chlorine or sodium hypochlorite) are used in addition to the flocculant (aluminum sulfate, polyaluminum chloride, polymeric flocculant, or iron flocculant). Also, in the water purification plant, proper chemical treatment is carried out while monitoring water quality conditions of raw water, purified water, and faucet water.
Then, if something unusual happens to the water quality of the raw water, measures are taken to more intensively inject water purification chemicals than under normal conditions. For example, when the concentration of a reducing material such as manganese, ammonium-nitrogen, or organic matter in the raw water increases, measures are taken to increase chlorine injection rate. Also, when increasing concentration of synthetic detergents or contamination with odor or phenols is sensed, powdered activated carbon treatment is usually carried out. In such a case, it is necessary to intensify chlorine treatment and coagulation sedimentation treatment in addition to injection of the powdered activated carbon.
An important point in water purification by the rapid filtration method is to form flocs which readily precipitate, by controlling the injection rate of the flocculant to an appropriate value according to the water quality of raw water. Flocculation treatment at an inappropriate injection rate will cause floc carryover from sedimentation basin or flocculation failure, resulting in problems such as increased head loss (filter resistance) of the filter basin, increased cleaning frequency, and leakage of very fine particles from the filter basin.
Also, activated carbon treatment for the purpose of removing soluble components such as organic matter or mold odor may adopt a combination of an activated carbon injection method and the membrane filtration method. Furthermore, treatment flows which incorporate treatment via injection of chemicals such as a flocculant are increasingly adopted because flocculation treatment is necessary depending on conditions such as a membrane type or permeation flux. With the membrane filtration method, a flocculant is added to the water to be filtered, to improve filterability and prevent clogging by increasing the size of very fine particles which could otherwise become fouling material.
A combination of feedforward control (hereinafter referred to as FF control) and feedback control (hereinafter referred to as FB control) is performed by a conventional chemical injection control method.
An appropriate flocculant injection rate varies depending on the source water quality, changing with the turbidity, alkalinity, pH, water temperature, and the like of raw water, and thus cannot be determined uniquely using raw water turbidity alone as an index. Therefore, conventionally the state of flocculation has been judged and the flocculant injection rate has been determined or controlled in water purification plants as follows.
Examples include a method which performs FF control based on an injection rate calculation formula which describes a relationship with appropriate flocculant injection rate using water quality such as the turbidity, pH, alkalinity, and water temperature of raw water as parameters. The calculation formula has been derived by an empirical method based on jar testing, supernatant turbidity in actual facilities, and/or the like. Examples of a developed version of this control system include a combination with FB control based on measured values of supernatant turbidity, and neuro/fuzzy logic AI control performed so as to approach results of jar testing conducted by an operator or operational performance of actual facilities.
Examples of background art documents which disclose FF control, FB control, or combinations thereof include Patent Literatures 1 to 3.
A chemical injection control method disclosed in Patent Literature 1 controls the injection rate of chemicals in real time based on the start time of agglomeration of particles in raw water.
A flocculant addition control method disclosed in Patent Literature 2 prevents excessive addition of flocculant by controlling the amount of flocculant addition based on the value of ultraviolet absorbance of a membrane filtrate produced by a membrane separation means.
A flocculant injection control method disclosed in Patent Literature 3 calculates optimal values of a flocculant injection rate, pre-alkali chemical injection rate, post-alkali chemical injection rate by multiple regression analysis and controls the amounts of injection of flocculant and alkali chemicals based on the injection rates.
On the other hand, in activated carbon injection control for the purpose of removing soluble components such as organic matter or mold odor, the injection rate of activated carbon is determined so as to obtain target water quality. However, it takes an awful lot of time to measure a trihalomethane precursor (hereinafter referred to as THMFP) or mold odor, and thus it is difficult to perform activated carbon injection control based on results of on-site measurements.
Thus, methods have been proposed which perform activated carbon injection control by predicting substances to be removed based on a statistical technique or by using alternative indices. For example, because the amount of THMFP production changes greatly with the water temperature and electrical conductivity of raw water, a method has been proposed which determines the injection rate of activated carbon corresponding to the amount of production by predicting substances to be removed based on a statistical technique (Patent Literature 4).
However, the conventional control techniques have the following problems.
When time variation in the water quality of raw water increases, a time delay occurs in FB control, making it difficult to follow a chemical injection rate sufficiently. Thus, it is important how to set the chemical injection rate appropriately in response to fluctuations in the water quality of raw water.
Also, in actual chemical injection control, even if there are fluctuations in the water quality of raw water, operation is run based on the chemical injection rate by providing a leeway in the amount of chemical injection so as to meet target treated-water quality even if there are fluctuations in the water quality of raw water. Moreover, when there are fluctuations in raw water quality, it is necessary to meet the target water quality by further increasing the degree of leeway. However, such a chemical injection control method involves injecting a more than necessary amount of chemicals, resulting in increased chemical costs. Especially when there are fluctuations in raw water quality, the impact of this becomes noticeable.